1. Field of the Invention
The present invention relates to a thermally processed image recording material. More precisely, the present invention relates to a thermally processed image recording material suitable for use in photomechanical processes for ultrahigh contrast images.
2. Description of the Related Art
There are known many photosensitive materials having a photosensitive layer on a support, with which image formation is attained by imagewise light exposure. Those materials include those utilizing a technique of forming images by heat development as systems that can contribute to the environmental protection and simplify image-forming means.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photomechanical processes from the standpoints of environmental protection and space savings. Therefore, development of techniques relating to thermally processed image recording materials for photomechanical processes is required, which materials enable efficient exposure by a laser scanner or laser image setter and formation of clear black images having high resolution and sharpness. Such thermally processed image recording materials can provide users with simpler and non-polluting heat development processing systems that eliminate the use of solution-type processing chemicals.
Methods for forming images by heat development are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klosterboer, xe2x80x9cThermally Processed Silver Systems Axe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Such thermally processed image recording materials comprise a reducible non-photosensitive silver source (e.g., silver salt of an organic acid), a photocatalyst (e.g., silver halide) in a catalytically active amount and a reducing agent for silver, which are usually dispersed in an organic binder matrix. While the photosensitive materials are stable at an ordinary temperature, when they are heated to a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced from the reaction of the reducible silver salt in the exposed areas shows black color and provides contrast with respect to the non-exposed areas, and thus images are formed.
Photosensitive materials for use in photomechanical processes are required to provide ultrahigh contrast images showing a xcex3 value of 10 or more. In order to form images of ultrahigh contrast, an ultrahigh contrast agent, i.e., nucleating agent is required.
Various ultrahigh contrast agents are disclosed in U.S. Pat. Nos. 5,496,695, 5,545,515, 5,635,339, 5,654,130, 5,705,324, Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) No. 11-119372, JP-A-11-109546, JP-A-11-231459 and so forth. However, photosensitive materials using the aforementioned ultrahigh contrast agents suffer from a problem that satisfactory levels are not achieved in those materials for all of Dmin, Dmax, high contrast and storage stability as products under a high humidity condition, and their improvements have been desired. Further, there are no cases in which relationship between pKa value of an ultrahigh contrast agent and various performance items.
In view of these problems of the prior art, an object of the present invention is to provide a thermally processed image recording material suitable for photomechanical processes that shows low Dmin, high Dmax, ultrahigh contrast and xcex3 value of 10 or more as well as superior storage stability.
As a result of the present inventors"" assiduous investigations, they found that an excellent thermally processed image recording material that provides the desired effects could be obtained by adding a compound represented by the following formula (1), which had a pKa value within a particular range, to a thermally processed image recording material as an ultrahigh contrast agent, and thus accomplished the present invention.
The present invention provides a thermally processed image recording material having at least one image-forming layer on one side of a support, which contains, on the side of the support, a silver salt of an organic acid, a reducing agent and at least one kind of a compound represented by the following formula (1): 
In the above formula, X and Y each independently represent an electron-withdrawing group and M represents a counter cation. The conjugate acid of the enolate anion in the formula (1) has a pKa value of 3.0-6.0.
In a preferred embodiment of the present invention, the material contains at least one kind of a compound represented by the following formula (A) having a molecular weight of 480 or more on the side of the support having the image-forming layer. 
In the above formula, R1 and R2 each independently represent a hydrogen atom or a monovalent substituent, X1 represents an oxygen atom, a sulfur atom or a nitrogen atom, Y1 represents a group represented as xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94C(xe2x95x90S)xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94C(xe2x95x90NR3)xe2x80x94 or xe2x80x94(R4)Cxe2x95x90Nxe2x80x94 where R3 and R4 each independently represent a hydrogen atom or a substituent, and Z1 represents a nonmetallic atomic group that can form a 5- to 7-membered ring together with X1 and Y1 However, R1 and R2 do not bind to each other to form a ring structure.
In another preferred embodiment of the present invention, the material contains a least one kind of a compound represented by the following formula (xcex1) on the side of the support having the image-forming layer. 
In the above formula, Z represents an alkyl group, an aryl group or a heterocyclic group, W represents an aryl group or an alkyl group substituted with an electron-withdrawing group, Z and W may bind to each other to form a ring structure, and M represents a counter cation.